Preparation of waste water containing sodium chloride for use in chlor-alkali electrolysis

ABSTRACT

The present invention relates to a process for the treatment of waste waters containing common salt, characterised in that, by a specific sequence of acidification, extraction, alkalisation and stripping steps, an aqueous common salt solution is obtained, which can be used directly in chlor-alkali electrolysis.

The present invention relates to a process for the treatment of wastewaters containing common salt, characterised in that, by a specificsequence of acidification, extraction, alkalisation and stripping steps,an aqueous common salt solution is obtained, which can be used directlyin chlor-alkali electrolysis.

Waste water containing common salt is obtained in many chemicalprocesses, for example in the interfacial polycondensation process forthe production of polycarbonate, or in the production of diphenylcarbonate, also by the interfacial polycondensation process, and manyother chemical reactions in which common salt is formed directly orindirectly (cf. e.g. Schnell, “Chemistry and Physics of Polycarbonates”,Polymer Reviews, volume 9, Interscience Publishers, New York, London,Sydney 1964, p. 33 ff.).

To purify these waste waters, many methods are already known, such ase.g. activated carbon adsorption, distillation, extraction orozonolysis. While it is true that the purified waste waters are thenfreed from most impurities, the waste water is nevertheless unsuitablefor introduction into the environment owing to the common saltremaining. It is particularly problematic, for example, if the wastewater is introduced into fresh water areas, which may still be used forthe drinking water supply.

The question therefore arises of how these waste waters could better beeliminated. One possible solution would be to use these waste waters inchlor-alkali electrolysis. This would mean that, firstly, theenvironment would not be polluted with the salt and secondly, resourceswould be spared and thus raw material costs also saved.

For use in chlor-alkali electrolysis, however, only waste waterscontaining virtually exclusively chlorides as anions are suitable. Wastewaters containing other anions and organic impurities must therefore besuitably treated beforehand.

Waste waters arising from polycarbonate or diaryl carbonate production,for example, also contain carbonates from phosgene hydrolysis inaddition to common salt concentrations of 2 to 20%. In addition, apartfrom these inorganic salts, organic impurities are also present. Thus,residues of phenols or bisphenols, catalyst and solvent are stillpresent. All these impurities would have to be reduced to a minimum tomake use in chlor-alkali electrolysis possible.

It is known from EP A1 0 396 790 that dilute solutions that are formedcan be treated by reactive extraction steps in such a way that arecyclable concentrate of certain components is obtained. However, nocomplete solution is disclosed for all solution streams that are formed.Also, nothing is said about a possible purification of a waste waterstream containing common salt for use in chlor-alkali electrolysis.

Similar methods of treatment by means of physical extraction steps arealso known to the person skilled in the art (cf. e.g. UllmannsEncyclopedia of Industrial Chemistry, volume B 3 page 6.3 to 6.6). Here,however, only dilute waste water streams are purified by extractionmethods in such a way that the impurities are converted to moreconcentrated solutions, which can then be disposed of substantially moreeasily or cheaply.

It is known from DE-A 195 10 063 that waste waters from reactionscontaining common salt, e.g. as arising from the interfacialpolycondensation processes for polycarbonate or diphenyl carbonatesynthesis, can be treated by means of a reactive extraction afteracidification in such a way that a solution suitable for introductioninto the environment is obtained. It is also pointed out that thissolution can be used in chlor-alkali electrolysis after suitableconcentration. However, the process described there is not suitable fordirectly providing a solution that is suitable for use in chlor-alkalielectrolysis. The load of organic residues still present after thisprocess would also be concentrated during a concentration step and thusthe solution would become unsuitable for chlor-alkali electrolysis. Evenin the solution obtainable by this process without concentration, theload of organic residues would still be too high to use the solution inthe membrane process for chlor-alkali electrolysis that is preferredtoday. Thus, DE-A 195 10 063 only discloses waste waters with a CODvalue of preferably <100 ppm, or at least 34 ppm in the examples. Thesewaste waters would not be suitable for use in chlor-alkali electrolysis.

These known processes for the treatment of waste process waterstherefore do not lead directly to a solution containing common salt thatis suitable for use in chlor-alkali electrolysis, particularly by themembrane process. There is also no indication that it could be possibleto achieve this with the aid of the technologies available up to thepresent.

On the basis of the prior art, therefore, the object arose of makingavailable an improved treatment process for waste waters containingcommon salt, which leads to solutions containing common salt that aresuitable for direct use in chlor-alkali electrolysis, and to as completea use as possible of the partial streams with the smallest possiblequantities of waste being obtained.

Surprisingly, it has now been found that waste process waters containingcommon salt can indeed be treated in such a way that the remainingcommon salt solution can be used directly in chlor-alkali electrolysisin that the waste process water

-   -   is acidified with HCI and degassed    -   then extracted with an organic solvent,    -   the aqueous phase is alkalised and    -   stripped with steam.

The process according to the invention achieves the following surprisingadvantages compared with the known processes of the prior art:

-   -   1. The salt solution obtained can be used directly in        electrolysis; concentrating is not necessary. In the case of        membrane electrolysis, salt purification is no longer necessary        and the water can be recycled.    -   2. Salt and water quantities are reduced.    -   3. In the case of diphenyl carbonate production, the diphenyl        carbonate residues present as an impurity in the waste water are        converted to phenol during the extraction.    -   4. The extracted phenols can be reused as a raw material in the        synthesis.    -   5. Only small quantities of waste water remain, which benefits        the environment.    -   6. The process can also be operated without the use of        components for reactive extraction.    -   7. The COD value achieved in the treated waste water is below 30        ppm and thus below the application limit of the COD process. The        value cannot therefore be determined accurately but it is very        low.    -   8. Content of phenolic impurities <1 ppm, of phenol <0.3 ppm, of        bisphenol below the limit of detection, catalyst residues <1 ppm        and organic solvent <1 ppm.

According to the process of the invention, the waste water from thereaction is first acidified with HCl, preferably with commercial 37%aqueous acid, to a pH of 1-5, preferably 3 to 4, especially preferably3. The carbonates are thus converted to carbonic acid, which escapes asa gas. It is possible for the carbonic acid to be recovered in order tobe converted to CO in a reformer. Phenolic anions are also converted tothe corresponding free phenolic compounds.

The acidic solution is then brought into contact with an extractingagent. Apolar organic solvents, such as e.g. methylene chloride,chlorobenzene or a mixture of the two, MIBK (methyl isobutyl ketone) orether, preferably methylene chloride, chlorobenzene, or a mixture of thetwo, can be used as the extracting agent. Alternatively, an insolublebase, preferably long-chained tertiary amines, such as e.g. alamine ortriisooctylamine, particularly triisooctylamine, can be used as thereactive extracting agent, dissolved in inert apolar organic solvents,such as e.g. petroleum fractions, Shell-Sol AB, for example, beingpreferred. However, physical extraction with inert organic solvents ispreferred. During this extraction, the phenolic compounds and otherorganic compounds are removed from the aqueous solution. This extractiontakes place in several, preferably 4-10, steps. Mixer-settlers orextraction columns, preferably extraction columns, particularlypreferably pulsed packed or sieve tray columns, can be used for thispurpose, cf. e.g. Perry's Chemical Engineering Handbook, McGraw Hill,New York, 1999, 15-44 to 15-46. A ratio of organic phase to aqueousphase of 5:1 to 1:5, preferably 3:1 to 1:3, especially preferably 1:2,is aimed for.

The organic extraction phase obtained is then re-extracted with anaqueous sodium hydroxide solution in a concentration of 1 to 30%,preferably 5 to 20% NaOH. The alkaline-aqueous phase is used asextracting agent here in a significantly smaller quantity in order toachieve the highest possible phenolate concentrations in thealkaline-aqueous phase. A ratio of aqueous sodium hydroxide solution toorganic phase of about 1:50 to 1:1000, preferably 1:400 to 1:1000, wouldsuffice for the extraction with aqueous sodium hydroxide solution. Theprecise ratios, however, depend on the concentration of phenol in theorganic phase to be worked up, as this is a reactive extraction in whichapprox. 1.1-1.5, preferably 1.2-1.3, particularly preferably 1.25 molNaOH per mol phenol have to be used. Accordingly, the quantities have tobe adapted to the concentration of phenol in the organic phase in eachcase. In order to achieve a miscible ratio, however, which does notexist with ratios of 1:50 to 1:1000, sodium hydroxide solution iscirculated, as a result of which an actual ratio of circulated sodiumhydroxide solution to organic phase of approx. 1:10 is achieved. Apartial stream is removed from the circulated sodium hydroxide solution,which is replaced by fresh lye in each case. The ratio of removedpartial stream to quantitative stream of the extracted organic phase nowcorresponds to the ratio mentioned above. The aqueous extract obtainedhere can be further treated to recover phenols.

A preferred method consists in carrying out the re-extraction withsodium hydroxide solution in two stages. In the first extraction step,extraction is performed as described above with an aqueous sodiumhydroxide/phenolate solution, which is formed from the removed partialstream of the second extraction step, with the addition of extra NaOH tore-establish a concentration of 1 to 30%, preferably 5 to 20% NaOH. Thepartial stream forming at this stage is fed directly to phenol recoveryand a corresponding quantity of sodium hydroxide solution from thesecond stage is fed back in as fresh lye, with the addition of extraNaOH to re-establish a concentration of 1 to 30%, preferably 5 to 20%NaOH. In the second extraction step, extraction is performed asdescribed above with NaOH at a concentration of 1 to 30%, preferably 5to 20% NaOH, the removed partial stream being replaced with fresh lyeand this partial stream, with the addition of extra NaOH to re-establisha concentration of 1 to 30%, preferably 5 to 20% NaOH, is fed into thefirst stage as fresh extracting agent. As the removed partial streamfrom the first stage, a concentrated, aqueous-alkaline solution of thephenolates is obtained, from which two phases are formed by simpleneutralisation with HCl, which can be separated in a simple separatingvessel. In this way, an upper phase containing about 90% of the quantityof phenol is obtained, which can either be used again in a synthesis(e.g. DPC) or otherwise disposed of. The other phase consists of anaqueous common salt solution, which is slightly loaded with phenol andis fed back into the waste reaction water to be worked up.

The content of phenolic compounds in the organic phase is reduced toless than 1 ppm by this re-extraction. The organic phase freed fromphenolic compounds in this way is fed back into the extraction of thewaste reaction water as an extracting agent. The two-stage re-extractioncan be designed e.g. in the form of a counter-current extraction. Thesere-extractions are preferably performed in a mixer-settler, e.g. asdescribed in Perry's Chemical Engineering Handbook, McGraw Hill, NewYork, 1999, 15-22 to 15-29.

The extracted waste process water containing common salt, largely freedfrom phenolic and other organic compounds, is now alkalised with aqueoussodium hydroxide solution at any concentration, e.g. 1-50% NaOH, to a pHof 7-13, preferably 8-12, and stripped with steam at 1-4, preferably2-3, particularly preferably 2.5 bar in a stripping column, cf. e.g.“azeotropic distillation” in Perry's Chemical Engineering Handbook,McGraw Hill, New York, 1999, 13-68 to 13-75. The quantity of steam tothe quantity of solution to be stripped is in a ratio of 1-5, preferably2-4, particularly preferably 3-3.5 to 100. In this step, both thecatalyst and the residual solvent are removed. The top gases from thecolumn therefore contain the catalyst and residual solvent, arecondensed and can be fed back into the synthesis reaction. The bottomproduct is a pure common salt solution, which can now be used directlyin chlor-alkali electrolysis.

The content of residual organics in the common salt solution treated inthis way is <0.3, preferably <0.1 ppm, bisphenols and catalyst residuescan no longer be detected and the residual content of organic solventsis <1 ppm, preferably <0.1 ppm.

All the steps in the process according to the invention are, unlessexpressly described otherwise, performed at temperatures below thelowest boiling point of the solvents used in each case and underatmospheric pressure. If necessary, however, the steps can also beperformed at temperatures above these temperatures with pressureadjusted accordingly at the same time.

The following diagrams are intended to illustrate the process accordingto the invention, and the re-extraction explicitly, without, however,limiting the subject matter of the present invention.

EXAMPLES

The following examples are intended to illustrate the present invention,but without restricting it:

Example 1

A waste water from diphenyl carbonate production contains 200 ppmphenol, 30 ppm ethylpiperidine (EPP), 2 ppm diphenyl carbonate and 0.25%sodium carbonate.

98 kg of this waste water are brought to pH 4 with 2 kg of 37%hydrochloric acid and degassed. The residual concentration of carbonateions is less than 200 ppm.

This solution is then extracted in an extraction column 5 metres inlength, 0.05 metres in diameter and with 50 sieve plates using half thequantity (weight ratio) of methylene chloride.

The phenol concentration in the waste water after the extraction columnis <200 ppb. The ratio of waste water to extracting agent (methylenechloride) is 2:1.

The resulting 50 kg of solvent, containing 400 ppm of phenol and 4-5 ppmof iphenyl carbonate, are then re-extracted with 250 g of 20% sodiumhydroxide solution in 2 mixer-settlers in a counter-current process. There-extract is neutralised with 241 g of 37% HCl. This solutionseparates, giving 19 g of organic phase (95% phenol) and 493 g ofaqueous phase (1% phenol) at pH 4. These 493 g of aqueous phase are fedback into the fresh waste water at the beginning of the extraction. 50kg of purified, water-containing solvent are fed back into theextraction.

100 kg of extracted waste reaction water from the extraction are thenstripped in a stripper with 3.15 kg of steam at 2.5 bar. 1.03 kg of EPPand methylene chloride-containing water remain as top distillate, whichcan be fed back into the synthesis. As bottom distillate, 102.3 kg of anaqueous common salt solution remain, with 15-18% common salt and <1 ppmEPP and <1 ppm methylene chloride.

The COD is 28 ppm and can therefore no longer be reproducibly measured,as the sensitivity of the method is insufficient. The high NaCl contentof the solution also leads to higher readings, so that the actual COD issignificantly lower.

1-10. (canceled)
 11. A process for purifying waste water containing oneor more chlorides, residual acids, residual bases or residual solventscomprising: a) acidifying the waste water, b) contacting the waste waterfrom step a) with an extracting agent in a manner such that an organicphase and an aqueous phase are generated, c) adding an alkaline materialto the aqueous phase generated in b) in an amount sufficient to causethat phase to have a pH of from 7 to 13, and d) stripping the treatedphase from step c) to obtain purified water.
 12. The process of claim 11in which the waste water to be treated was generated from an interfacialpolycondensation process for production of a polycarbonate or a diphenylcarbonate.
 13. The process of claim 11 in which carbonates are removedfrom the waste water to be treated prior to step a).
 14. The process ofclaim 13 in which carbonate removal is achieved by acidifying anddegassing the waste water.
 15. The process of claim 11 in which organiccompounds are removed by extraction with solvent.
 16. The process ofclaim 11 in which the extracting agent used in step b) is a reactiveextracting agent.
 17. The process of claim 11 in which step b) iscarried out in a column.
 18. The process of claim 11 in which at leastone phenolic compound is recovered by re-extracting the organic phasegenerated in step b) with an aqueous alkaline solution to generate asecond organic phase and aqueous phase and neutralizing the secondaqueous phase.
 19. The process of claim 18 in which the re-extracting isconducted in one or more mixer-settlers.
 20. The process of claim 18 inwhich the re-extracting is conducted as a countercurrent extraction. 21.The process of claim 18 in which the re-extracting is conducted intwo-stages.